The BBC said the filming was "standard practice" for natural history shows.

"This particular sequence would be impossible to film in the wild," a BBC spokesperson said.

"The commentary accompanying the sequence is carefully worded so it doesn't mislead the audience and the way the footage was captured is clearly explained on the programme website."

'Not falsehood'

Speaking to ITV1's This Morning programme, Frozen Planet presenter Sir David Attenborough said: "If you had tried to put a camera in the wild in a polar bear den, she would either have killed the cub or she would have killed the cameraman.

He added that an explanation about the animal park footage would have ruined the atmosphere of the sequence.

"It's not falsehood and we don't keep it secret either," said Sir David.

In the episode, broadcast on 23 November, the camera follows a female polar bear in the Arctic as Sir David comments: "She starts to dig a shallow nest... once the snow here is deep enough, she'll dig down to make a den. She'll then lie waiting for her cubs to be born as winter sets in."

Image caption
Sir David travelled to both polar regions with the film crew

Later, the film cuts to a mountainside: "On lee-side slopes beneath the snow, new lives are beginning," Sir David narrates.

Footage of the newborn cubs filmed at the animal park is then screened.

The den was actually created by humans before the polar bear entered.

The BBC said the narration "talked in general about polar bears in the wild rather than the specific cubs shown".

In the online clip, posted on 7 November - before the episode in question was broadcast - producer Kathryn Jeffs explains: "The problem for us is that they (the polar bears) do it (give birth) underneath the snow in these dens of ice, and there's absolutely no way that we could get our cameras down there.

"It would be completely impractical and you wouldn't want to disturb the polar bears by getting that close," she added.

The BBC programme features 10-minute segments at the end of each episode explaining how film crews got up close to the wildlife, but the polar bear cub footage was not included here.

Media regulator Ofcom said they have received "fewer than a handful" of complaints about Frozen Planet.

The organisation will now decide whether they will investigate further or it is a matter for the BBC Trust.